A liquid crystal display (LCD) includes a pair of panels with field generating electrodes and a liquid crystal layer with dielectric anisotropy disposed between the two panels. An electric field is formed in the liquid crystal layer by using the electrodes, and the desired images are generated by adjusting the electric field to control the light transmittance through the liquid crystal layer. The LCD devices include flat panel display (FPD) devices, which frequently come in the form of TFT-LCDs that use thin film transistors (TFTs) for pixel control.
TFT-LCDs, which were used primarily as computer monitors in the past, are becoming utilized more for entertainment display screens such as television screens. As a result, it has become more important for TFT-LCDs to display quality moving images. However, because TFT-LCDs were traditionally not used to display fast moving images, some improvement is needed for the signal control technology in these devices. Currently, the liquid crystal molecules do not respond to the applied electric field fast enough to display clean fast-moving images. It takes a certain length of time for the liquid crystal capacitor to be charged to a target voltage. When the difference between the target voltage and the previous voltage is large, the liquid crystal capacitor may take a longer than desired length of time to reach the target voltage. A “liquid crystal capacitor” refers to the pair of electrodes that generate the electric field and the liquid crystal layer disposed therebetween.
One of the solutions for the problem of long liquid crystal layer charge time is dynamic capacitance compensation (DCC). The DCC method entails applying a modified voltage, which is higher than a target voltage, to the liquid crystal capacitor to take advantage of fact that the response time decreases as the voltage across the liquid crystal capacitor increases. FIG. 1 is a plot of the luminance level as a function of time in a conventional display device. Time is expressed as the number of frames. Using plots such as the one shown in FIG. 1, the display device determines what modified gray signal to apply to the liquid crystal capacitor. The plot of FIG. 1 illustrates a case where a previous voltage is “0” and the target voltage at frame 1 is “128.” According to the plot, a modified gray signal of “208” should be applied to bring the previous voltage of “0” to the target voltage of“128” within one frame. However, the plot also shows that the luminance drops back down by over 10% in the next frame before gradually climbing back up to the desired luminance level. This drop in the luminance level followed by a gradual climb causes “flickering” in the displayed images. The “flickering” phenomenon is especially bad where gray level voltages are low.
When using a computer aided design (CAD) program to draw an object, the program can be operated in a wire frame mode that depicts the object as a wire frame with lines representing a three-dimensional object. When the object is moved across the screen in the wire frame mode or zoomed in or out, some flickering is seen on the screen. This flickering phenomenon, called “wire frame flickering,” is particularly severe in a patterned vertically aligned (PVA) mode LCD having cutouts at the field generating electrodes.
FIG. 2 depicts a test screen 20 that is used to check the performance of a liquid display device. As shown, the test screen includes a rectangle 22 (which is usually red) on a gray screen 24. When the rectangle 22 is moved diagonally across the gray screen 24, in the direction indicated by an arrow 25, cyan-colored artifacts 26 briefly appear near two of the corners. The cyan-colored artifacts appear in the regions where the underlying colors have to rapidly change from gray to red and back to gray as the red rectangle is moved. When the rectangle moves, the gray signals for the green and blue pixels in the regions that are touched by the corners during the move have to rapidly switch from 128 to 0, then back to 128. When the gray signal changes from 0 to 128, the DCC-modified signal that is applied is 208. As a result of applying this modified signal, an overshooting of the luminance level occurs as shown in FIG. 3. The luminance levels in the green and blue pixels become higher than what is desired, making the cyan-colored artifacts 26 appear.
The undesirable appearance of cyan-colored artifacts 26 indicates that DCC-based modification does not always provide the desired result. When using DCC, the modified signal is selected based on the assumption that the previous signal has been stabilized. Thus, in cases like above where the “0” signal is sustained only for a brief moment (i.e., one frame) and not stabilized, applying the signal 208 results in overshooting.
The above-described overshooting phenomenon can be explained in reference to the liquid crystal capacitor. FIGS. 4A, 4B, and 4C are plots of liquid crystal capacitance as a function of the gray signal. More specifically, FIG. 4A shows the liquid crystal capacitance (C128) when a gray voltage 128 (V128) is applied. FIG. 4B shows the liquid crystal capacitance when a gray voltage 0 (V0) is applied when the display device is in the condition illustrated in FIG. 4A. When V0 is applied, the charge in the pixel is Q0=C128×V0. The liquid crystal molecules reorient themselves according to V0, which in turn changes the liquid crystal capacitance. As for the TFTs, each TFT turns on for only a fraction of the time designated for one frame, and remains off for the remainder of the frame. When the TFT is turned off, the Q0 should remain constant. Thus, when the liquid crystal capacitance changes, the gray voltage has to be adjusted to maintain a constant Q0. FIG. 4C shows the liquid crystal capacitance at the end of the frame where V0 was applied. At the end of the frame, the liquid crystal capacitance has changed to CG′ and the gray voltage has been adjusted to VG′, from V0. Applying the modified signal 208 in this state usually means a signal that is unnecessarily high is being applied. Thus, the overshooting creates the cyan-colored artifacts 26.
A method of reducing the liquid crystal response time without causing quality-degrading consequences (such as flickering or overshooting) is desired.